WASHINGTON — Whites are now in the minority in nearly one in 10 U.S. counties. And that increased diversity, fueled by immigration and higher birth rates among blacks and Hispanics, is straining race relations and sparking a backlash against immigrants in many communities.

"There's some culture shock," said Mark Mather of the Population Reference Bureau, a Washington-based research agency. "But I think there is a momentum building, and it is going to continue."

As of 2006, non-Hispanic whites made up less than half the population in 303 of the nation's 3,141 counties, according to figures the Census Bureau is releasing Thursday. Non-Hispanic whites were a minority in 262 counties in 2000, up from 183 in 1990.

The Census Bureau's report has population estimates by race and ethnicity for every county in the nation. They are the first such estimates since Hurricane Katrina hit the Gulf Coast in 2005, scattering hundreds of thousands of people.

The biggest changes in were in Orleans Parish, La., home to New Orleans. The share of non-Hispanic whites in Orleans Parish grew from 27 percent in 2005 to 34 percent in 2006, while the share of blacks dropped from about 68 percent to 59 percent.